I bought the spandex at a fabric store.
There was only one choice in that color so I didn't pay attention to the
type. In one dimension it stretches a little more
than twice the original length, in the other dimension it stretches a
little less than twice the original length. We have
also used it to make a gravitational well and to help visualize
equipotential lines.

The strip I cut for mine was 1.5" wide and
80% of the distance around the pulleys (the long dimension coincided with
the stretchier orientation). The ends were cut at a 45 degree
angle, joined end to end, zig-zag stitched together, then straight
stitched back and forth across the seam for a distance of ~3/4" on
either side. All stitches were at max stitch length to minimize
fabric distortion and weight. The loose ends were glued down with
tiny amounts of rubber cement (Barge).

You may want to experiment with the length since
I recall it could not run at full speed (too much flapping). We ran
it up to 90% of full speed and it worked as well as it ever did, so I
never bothered to tighten it up. If I make another one, it will be
a little bit shorter. But since we have a new machine (Science
First) which is quite a bit better, I may not get around to that for
awhile...

Jerry

At 12:13 PM 9/26/2003, you wrote:

Jerry,

Sewing black spandex seems simpler than glueing strips of latex.
Any

particular brand or composition of the spandex - or is this Spandex
(a

"brand" name)? And if so do they have different
types? Do you have a

"formula" for how short you cut it to allow for stretching when
it is

installed?

Dick Berg

On Fri, 26 Sep 2003, Jerry DiMarco wrote:

> I made a belt for our old Winsco N100V
VDG out of a strip of black spandex back in the previous century (Oct of
'97).

> Last spring it had to be retired, it just wanted to curl in from the
edges and run off the end of the roller. So that's 5

> 1/2 years, compared to ~1 yr for the replacement belts. It
requires a little sewing skill. If you're interested in
details,

Just got a belt custom made by Durabelt. http:\\www.durabelt.com It should
last 10 years if stored under the right conditions, I was told. The belt
drive must be supported by bearings independent of the motor, otherwise the
higher tension in these belts will stop the motor. (I.e. the motor drive
shaft can't directly be a support for the belt drive shaft.)

But the VDG still does not generate much charge - and as it stays on for a
couple minutes the charge decreases - so it might be leaking somewhere?

I replaced the disintegrated cloth on the bottom roller with what I think is
an acrylic/polyester cloth. I think the two rollers need to be different
materials - hence the cloth, is that right? What should the cloth be, or do
I even need it? What is the max distance that the combs can be from the
roller?

I have an older Cenco VDG - about 26 cm sphere and column about 56 cm high.
I should be able to get sparks longer than 10 cm, shouldn't I?

Wolfgang is right. I believe the phrase "Most powerful magnet..." was
used in reference to Nd magnets in general. The "Giant Neo" that David
has is 7/8" x 1", and it sounds like people doing this demo are using a
flatter disc shape. Right?

I agree. I have a 0.5" diameter x 0.5" high cylindrical magnet that
takes almost 30 sec to "drop" down a 5' length of copper pipe from the
hardware store. I have to let people put there ear to the pipe and
listen to the quiet clinking sound to convince them it isn't just stuck.

BTW, I had our shop mill a 1/8" slot down the length of the tube. It
speeds things up, but there is still plenty of drag. More like 20 sec to
fall 5', and you can see the thing the whole way.

Stephen Bannasch wrote:

>At 8:54 AM -0500 9/26/03, Paul Nord wrote:
>
>
>>yes.
>>
>>The conductivity increases a lot with thickness. A thin-walled copper pipe that you might use for plumbing will show no drag when you drop a magnet through it.
>>
>>
>>
>
>Not my experience. When I drop a 0.7" spherical neodymium magnet down through a two foot section of 1" copper standard plumbing pipe the magnet takes about 2s to transit the pipe. It is the strangest thing to watch from above.
>
>
>

I agree. I have a 0.5" diameter x 0.5" high cylindrical magnet that takes
almost 30 sec to "drop" down a 5' length of copper pipe from the hardware
store. I have to let people put there ear to the pipe and listen to the quiet
clinking sound to convince them it isn't just stuck.

BTW, I had our shop mill a 1/8" slot down the length of the tube. It speeds
things up, but there is still plenty of drag. More like 20 sec to fall 5',
and you can see the thing the whole way.

Stephen Bannasch wrote:

cite="midp06002031bb9a16447a9a@%5B4.19.234.164%5D">

At 8:54 AM -0500 9/26/03, Paul Nord wrote:

yes.

The conductivity increases a lot with thickness. A thin-walled copper pipe that you might use for plumbing will show no drag when you drop a magnet through it.

Not my experience. When I drop a 0.7" spherical neodymium magnet down through a two foot section of 1" copper standard plumbing pipe the magnet takes about 2s to transit the pipe. It is the strangest thing to watch from above.